Our research focuses on the underlying mechanism of cloacal septation. The putative urorectal septum divides the cloaca along dorsoventral axis. The dorsal compartment forms the rectum and anus, and the ventral urogenital sinus gives rise to the bladder and urethra. Cloacal septation is also linked with formation of the external genital tubercle and urethral tube; as a result, cloacal septation defects are almost always coupled with genital tubercle anomalies. Principle questions regarding cloacal septation, such as the embryonic origin and molecular determinants of the urorectal septum, remain poorly understood. These are important questions because lack of the fundamental knowledge is a major barrier to understanding the etiology of congenital urogenital birth defects, which are among the most common in humans. In spite of various existing models and interpretations, our fundamental understanding of cloacal morphogenesis and pathogenesis remains very limited. Our immediate objective is to understand the molecular and cellular basis of cloacal septation with an ultimate goal of applying this basic knowledge to help reduce urorectal-related diseases. Based on our published and preliminary findings, we have hypothesized that the Sdx1 domain is required for cloacal septation and urogenital development. The Sdx1- expressing mesenchymal cells and lineages are restricted and asymmetrically localized next to the developing cloaca. To our knowledge, this is the first time that a molecularly defined discrete domain has been identified and implicated in cloacal morphogenesis. In our opinion, this hypothesis is innovative because it represents a substantial departure from existing models. To determine the functional significance of the Sdx1 domain, we have designed three complementary aims: to functionally characterize the Sdx1 domain; to determine if Dkk1 inhibits formation of the Sdx1 domain; and to determine if endodermal Shh signal induces formation of the Sdx1 domain. The expected contribution of the proposed research is the advancement of our basic understanding of cellular and molecular mechanisms underlying the cloacal septation process. This contribution would have an important positive impact on fields of developmental biology and human congenital urorectal diseases. Future studies of genetic and environmental regulators of the Sdx1 domain would most likely lead to a better understanding of the underlying mechanism(s) and novel targeting and/or preventative strategies for congenital urorectal disease. PUBLIC HEALTH RELEVANCE: This proposal is relevant to public health because understanding basic developmental biology of lower urinary tract is the first of many steps towards a better understanding of etiology of and development of novel preventative strategies of congenital urogenital birth defects, which are among the most common in humans. The proposed research is relevant to the part of NIH's mission in pursuing fundamental knowledge to extend healthy life and to reduce the burdens of illness and disability.